Known electrical discharge machining apparatus typically includes a plurality of parallel wire-like electrodes, slidably held in a cartridge and passing through a nose guide to engage the workpiece that is to be machined. The cartridge is mounted for linear movement with respect to the nose guide in a direction parallel to the electrodes, and the nose guide is mounted for movement towards and away from the workpiece. The cartridge is equipped with a clamp that can engage the electrodes and prevent them from sliding relative to the cartridge.
During the machining operation, the nose guide is placed adjacent to the workpiece and, with the clamp engaged, the cartridge is moved forward, so that the electrodes slide through the nose guide and into the workpiece, machining holes as they advance. Power is provided for the electrical discharges through one or more contact plates on the cartridge, against which the electrodes are urged by the action of the clamp.
The tips of the electrodes become eroded as machining proceeds and so means are provided for refeeding the electrodes forward from the cartridge between machining operations to replace the lengths of electrode that have been lost by erosion and realign the electrode tips. The method of refeeding employed must feed different electrodes by different amounts in order to compensate for unequal erosion rates and a number of methods have been suggested to cope with this problem.
British patent number 1569561 discloses an electrical discharge machining apparatus wherein the cartridge includes a friction pad in engagement with the electrodes to resist their sliding motion. At the end of a machining operation, a clamp on the nose guide is engaged with the electrodes to hold them fixed while the cartridge is retracted from the nose guide. Then, for refeeding to occur, the clamp on the nose guide is released and the cartridge is advanced once more. The friction pad on the cartridge urges the electrodes forward in conjunction with the movement of the cartridge, overcoming the friction naturally occurring in the nose guide, until their tips reach the workpiece. Then the forward movement of the electrodes is prevented and the friction pad slides over them as the motion of the cartridge continues.
This method has the disadvantage that it is difficult to choose a coefficient of friction between the friction pad and the electrodes that allows the electrodes to be positively driven forwards and yet does not cause them to buckle during the slipping motion. The electrodes may be as little as 0.1 mm in diameter and so buckle very easily.
British patent number 1571666 discloses a similar apparatus wherein the electrodes are urged forward during the refeeding operation by means of rollers until they reach the workpiece, but there is the same problem of obtaining a suitable coefficient of friction with the rollers to prevent buckling of the electrodes.